Answer

- Download the CSS file http://bit.ly/2ufAoVj (The download is no longer available as the theme will be included, in a backwards-incompatible way, in the upcoming FileRun update.)
- Save it to "customizables/custom_theme.css"
- Set the "Color theme" to "Custom", from FileRun's control panel, under "Branding"
- Give feedback and report problems, kindly :)

- Download the CSS file http://bit.ly/2ufAoVj (The download is no longer available as the theme will be included, in a backwards-incompatible way, in the upcoming FileRun update.)
- Save it to "customizables/custom_theme.css"
- Set the "Color theme" to "Custom", from FileRun's control panel, under "Branding"
- Give feedback and report problems, kindly :)

Good question. So far it's a global setting, but feel free to post a feature request if you would find it useful to have a per-user option.

It's in the comment bellow. I've set it now as an answer, so you can't miss it, at the top of the page, right under the original post.

The download is no longer available as the theme will be included, in a backwards-incompatible way, in the upcoming FileRun update.

The dark theme is now included as standard feature in the available FileRun update.

Hope this topic is a good place to report issues with the dark theme:
When using the dark theme and a login page custom page background, the login box's color is changes to white, making everything look weird and text unreadable.

Shouldn't it stay dark grey?

Indeed, that isn't right.
It will be fixed with the next update. In the meantime, replace "css/theme_dark.css" with this one.

Just wanted to say that File Request and Upload page need to follow set theme.
Was easy to fix by editing the html files but should be updated by someone with access.
Customer support service by UserEcho